San Francisco  In less than a day, an electrical storm unleashed nearly 8,000 lightning strikes that set more than 800 wildfires across Northern California - a rare example of "dry lightning" that brought little or no rain but plenty of sparks to the state's parched forests and grasslands.

The weekend storm was unusual not only because it generated so many lightning strikes over a large geographical area but also because it struck so early in the season and moved in from the Pacific Ocean. Such storms usually don't arrive until late July or August and typically form southeast of California.

"You're looking at a pattern that's climatologically rare. We typically don't see this happen at this time of summer," said John Juskie, a science officer with the National Weather Service in Sacramento. "To see 8,000, that's way up there on the scale."

Thousands of firefighters battled the blazes Tuesday from the ground and air. The lightning-caused fires have scorched tens of thousands of acres and forced hundreds of residents to flee their homes, though few buildings have been destroyed, said Daniel Berlant, a spokesman for the California Department of Forestry and Fire Protection.

"It's just extremely, extremely dry," Berlant said. "That means any little spark has the potential to cause a large fire. The public needs to be extra cautious because we don't need any additional wildfires."

Despite the many lightning strikes that hit the ground on Saturday alone, the weekend thunderstorm brought little precipitation because the rain evaporated in hot, dry layers of the atmosphere before it hit the ground, Juskie said.

The lightning storm struck California when the state was experiencing one of its driest years on record. Earlier this month, Gov. Arnold Schwarzenegger declared a statewide drought and directed agencies to speed up water deliveries to drought-stricken areas. Many communities have adopted strict conservation measures.

From San Francisco to Los Angeles, cities have only seen a tiny fraction of the rainfall they normally receive at this point in a typical year.